This invention relates to an oil composition and, more particularly, to an oil composition which undergoes thermal deterioration to a lesser extent when employed at elevated temperatures and which also undergoes oxidative deterioration to a lesser extent when employed under an oxidating atmosphere.
Lubricating oils are employed at present in a wide field of appliations, and are frequently subjected to extremely high temperatures. That is, although a system to be lubricated is seldom at an elevated temperature in its entirety, it occurs frequently that the system is subjected locally to elevated temperatures. For this reason, there are known lubricating oils employed as a thermal medium oil, such as, for example, mineral lubricating oil, diphenyl, diphenyl ether, alkylbenzene, alkylnaphthalene or 1-phenyl-1-xylylethane. These lube oils are deteriorated in properties owing to heat while producing carbonaceous materials and sludges in combination with other factors, thus damaging the equipment. Although thermal stability is required of these lube oils employed for these applications, deterioration and formation of carbonaceous materials unavoidably occur under hostile conditions. For example, in the case of an engine oil, the inside of the cylinder is heated locally to an elevated temperature, due to heat of friction produced between the piston and the cylinder in the inside of the engine or to insufficient removal of combustion heat evolved by fuel combustion. Thus, the carbonaceous materials are produced in the inside of the cylinder to cause the sludge formation and engine troubles. Recently, attempts have been made to elevate the temperature in the engine by employing new materials, such as ceramics, as the engine material For this reason, more stringent demands for thermal stability tend to be placed on the engine oils and it is becoming a more and more crucial task to prevent thermal deterioration of lube oils and to prevent formation of carbonaceous materials and sludges. The same tendency may be seen in the case of a turbine oil and attempts have been made to adopt a ceramic gas turbine for motor cars through evolution of new materials. Inasmuch as the gases at higher than 1200.degree. C. are introduced in this case into the turbine, more stringent demands for thermal stability are placed on the lube oils. Thus, it is becoming a crucial task to evolve a lube oil free from formation of carbonaceous materials or from thermal deterioration at elevated temperatures.
On the other hand, poor oxidation stability may be mentioned as one of the most vulnerable points of lube oils. Thus, not only the physical properties of the lube oils are changed gradually due to transmutation caused by oxidation, but the formation of sludges by polycondensation of oxidation products or corrosion of metals caused by formation of oxidation products occur unavoidably. For preventing these inconveniences, recipe oils containing anti-oxidation agents are used for normal usages, and a variety of anti-oxidation agents have been proposed for this purpose. However, these anti-oxidation agents have some or other weak points or defects and almighty anti-oxidation agents have not been evolved to date. The functions required of the anti-oxidation agents include, in addition to the properties of preventing oxidation under normal operating conditions, compatibility with base oils and the properties of not affecting the physical properties of the base oil, not impairing the function of other ingredients and of not forming harmful materials, such as sludges, due to deterioration or transmutation of the anti-oxidation agents themselves. None of the existing anti-oxidation agents satisfies these requirements simultaneously, while constraints are imposed on their usages and composition ratios. It is therefore necessary not only to make an endeavor to evolve almighty anti-oxidation agents, but to evolve variegated anti-oxidation agents having various functions, and to devise methods for using them to the best advantage.
On the other hand, a variety of synthetic oils have been evolved for usages not met by the mineral lubricant base oil. Of these, the synthetic hydrocarbon oils, typified by poly-.alpha.-olefins, are superior in various properties relating to lubrication, but are lower than the mineral oils in oxidation stability, so that they need be admixed necessarily with anti-oxidation agents.
In addition, the lube oils are not deteriorated solely by heat or by oxidation, but are deteriorated in a majority of cases by the action of both the heat and the oxidation. More specifically, the heat treating oil, used as the metal working oil, is required to be superior in oxidation stability, safety, cooling or quenching properties and brightness, when used as the quenching oil.
It has been known that the cooling properties among these properties may be suitably adjusted by adding an oil-soluble high molecular material into the oil, while the brightness may be improved by addition of a small amount of organic acids or esters. A majority of the presently marketed heat treating oils essentially contain petroleum and occasionally contain these materials.
However, since these oils are exposed to elevated temperatures, they undergo deterioration in properties, as mentioned hereinabove, to produce carbonaceous materials, which are then affixed to the surface of the workpiece to impair the brightness. The oils are similarly affected in their cooling properties so that the desired quenching properties may not be obtained, thus necessitating complicated management operations, such as changing the quenching conditions or replacing oils. On the other hand, the electrical discharge machining oil has come to be employed in a number of applications, and has centered about die machining, as a metal working oil for electrical discharge machining, which allows for precise machining by taking advantage of the precision of an artificially established electrical discharge phenomenon. This electrical discharge machining oil is subjected locally to elevated temperatures and thereby thermally cracked in part to produce thermally cracked products which then undergo polycondensation to produce the carbonaceous materials unavoidably. When contaminated by these carbonaceous materials, the machining oil may aggravate the operability or enlarge the discharge gap. Hence, it becomes necessary to keep the machining oil clean by filtration through a filter. However, the carbonaceous materials produced in such system generally contain small-sized particles having the particle size of not more than 1 micron, so that a highly strict demand is placed on the filter, while a complex equipment and a laborious operation are necessitated. Hence, the demand has been raised, such as those described above, especially the demand for an oil composition that is invulnerable to thermal deterioration and deterioration by oxidation.
On the other hand, hydrogen-donating materials have been known for many years, and frequently employed for liquefaction of coals, cracking of heavy oils or visbreaking. While tetrahydronaphthalene is most representative hydrogen-donating material, it has also been well-known that a hydride of a compound having at least two condensed aromatic rings or a compound having both an aromatic ring and a five-member ring also exhibits hydrogen-donating properties. However, there have not been made attempts for or reports on tests aimed at applying these hydrogen-donating materials for prevention of oxidation or formation of carbonaceous materials on the occasions of employing lube oils.